Insulated gate transistors have been proposed in which an insulated gate is used to control the conductance of an inherent bipolar transistor. An example of one such device is shown in FIG. 1A of this application and the equivalent circuit derived therefrom is disclosed in FIG. 1B. FIG. 1A is based on FIG. 14 of U.S. Pat. No. 4,443,931 to Baliga et al. for "Method of Fabricating a Semiconductor Device With a Base Region Having a Deep Portion". In these structures, the base of the inherent bipolar transistor is coupled through a MOS channel to the collector which becomes the base drive current supply. Thus in these prior devices, the base drive current is derived from the collector region. The base drive current is the MOS channel current. In the insulated gate structures heretofore known, while the inherent transistor has been activated, the conductivity of the base region of the inherent transistor has not been fully modulated due to a lack of sufficient base drive current. More particularly, in typical insulated gate transistors, the inherent bipolar transistor is believed to operate at between 40 and 60% of its capacity. Specifically, the conductivity modulation of the device shown in FIG. 1A is reduced in the region above the P-base, which region is referred to as the so-called JFET region. This reduced conductivity introduces an unwanted voltage drop in the JFET region. While an additional implant in the JFET region can reduce the voltage drop, the additional implant will also reduce the breakdown voltage and thus lowers the upper limit of the voltage drop which can be supported in the JFET region.
Thus an unfulfilled need exists for an insulated gate semiconductor device which fully utilizes the inherent bipolar transistor structure under normal operating conditions and is not limited by structurally imposed limitations upon forward drop.